Device for analyzing a fluid medium

ABSTRACT

A fully functional analyzing unit is included within the fluid-tight housing of a dialyzer which may be immersed in the medium to be analyzed. An opening in the housing is closed by a dialysis membrane. A channel defining body cooperates with the membrane to define a flow channel. The unit includes a carrier fluid reservoir and a carrier pump for generating a flow of carrier fluid through the flow channel. By dialysis via the membrane, the flow of carrier fluid is transformed into a flow of sample fluid which is received in a reaction channel. Reagent fluid from at least one reagent reservoir is delivered to the reaction channel by at least one reagent pump, and a detecting device is coupled to the reaction channel for detecting a reaction product originating from a reaction between the reagent fluid and the sample fluid and for generating a corresponding detection signal. Effluent from the reaction channel is received in a waste reservoir. Average volume flow in the reaction channel during operation is less than 100 μl/min, allowing for at least 30 days of uninterrupted operation. The device is suitable for in situ real-time measurement of plant nutrient salts in process waters of waste water treatment plants.

The present invention relates to a device for analyzing a fluid medium,in particular a liquid.

Fluid analyzers may be used for controlling chemical and biologicalprocesses such as the treatment of sewage water. It is desirable, forexample, to lower the concentrations of nutritive salts, such asnitrogen and phosphate salts, in the effluent from waste water treatmentplants. Proper control of the biological processes in the plant isrequired. It is therefore advantageous to be able to measure the amountsof phosphate, nitrate and ammonium in the waste water as they, amongstother variables, influence on or inform about the biological processes.

Numerous patents deal with the object of analyzing fluids, especiallyliquids, for the presence of various analytes. The measurement methodscan in principle be divided into three groups:

(1) Methods in which a sample is taken out discontinually, filtered andanalyzed;

(2) On-line methods; methods in which a sample is pumped continually outof the bulk process fluid, filtered and then at regular intervalsautomatically analyzed;

(3) Methods being carried out in situ. Sampling and analyzing gear isentirely or partly immersed in the medium to be analyzed, or sampling isdirect and analysis is carried out so close to the process, that thetime between a sampling and the development of the analysis result isshort enough to allow reliable, real-time control of the process.

An analyzing system for use in process control applications shouldenable the user to take immediate precautions; for example in wastewater treatment, precautions against a suddenly increasing content ofphosphates in the fluid medium. The methods in group (1) however arepredominantly carried out in the laboratory which inevitably entails adelay in time from sample collection to actual analysis.

Moreover, as water samples are often analyzed spectrophotometrically,long transfer distances may present a further problem because thecontinuing biological activity in the samples tends to render them lessrepresentative. Even if the samples are transported quickly from thesampling site to the laboratory, the analysis results are somewhatuncertain because of problems relating to background turbidity in thesamples.

Group (2) above includes UV measurements as well as ion selectiveelectrodes and segmented flow analysis (SFA). The so-called flowinjection analysis (FIA), belongs to group (2) as well as to group (1).

The segmented flow analysis (SFA) method was first described in U.S.Pat. No. 2,797,149 and No. 2,879,141, the basic principle being that thesamples to be analyzed are separated from one another by air. Arefinement of this technique comprising a fluid handling system isdescribed in U.S. Pat. No. 4,853,336. This system is particularly usefulfor mixing liquid samples with previously separated processing liquids,such as reagents or diluents, in continuous flow analyzers. The systempermits the delayed on-line mixing of different components of ananalysis mixture, such as samples with reagents or diluents, as well asmixing and interaction of such components in a single conduit.

The basic FIA principle is outlined in U.S. Pat. No. 4,022,575 and No.4,224,033. A metered amount of the sample is introduced in a movingliquid carrier flow, constituting a well-defined zone, the volume andgeometry of which should be strictly reproducible. The sample zonewithin the carrier flow is transferred through an analysis module anddetected in a suitable detection cell. In FIA the sample may beintroduced directly in a predetermined amount, optionally using a valve,or it may be introduced using a system of magnetic valves, see e.g. U.S.Pat. No. 4,177,677.

Flow injection analysis requires that sample volumes be metered withgreat accuracy. This problem is addressed in EP published application107 631 which describes integrated microconduits for flow analysiswherein a miniaturized system of channels is formed in a monolithicstructure. A channel section is designed to be switchable between flowpaths, thus allowing the metering of a sample volume by placing it inthe switchable channel section while switched into sample flow, and thenswitching the channel section into analysis flow to process the samplevolume in a batch process.

An example of an arrangement belonging to group (3) as defined above isa polarography cell, the so-called Clark cell, for direct measurement ofthe proportional quantity of a substance in a composition. This isdescribed in U.S. Pat. No. 2,913,386. The cell includes a tubular bodyhaving a membrane-covered cavity wherein an anode and a cathode arearranged in a predetermined fixed spatial relation. The cavity is filledwith an electrolyte. The space between the electrodes defines a "bridge"through which ions are transferred while chemical reactions take placein the electrolyte. The electrolyte is consumed in the chemical reactionand needs to be replaced frequently. The cell is suited to determinee.g. oxygen, SO₂ or CO₂ in liquids, gases or solids.

A further example belonging to group (3) above is an analyzer designatedAPP (Automatic Pump Photometer) which has been constructed by MEMeerestechnik-Elektronik GmbH, see the document DE C1 38 22 788. Thisdevice is specifically designed for in situ use in water to takesamples, analyze the samples directly and store the results of themeasurements. The APP analyzer is able to detect changes inconcentrations of given substances within relatively short intervals(10-30 min.), the measurable substances being e.g. ammonium, nitrate,nitrite, phosphate, silicate, sulfide, cyanide and heavy metals. Thecentral part of the APP analyzer is a reciprocating pump which servesalso as a reaction cell and a cuvette and which aspirates the sample aswell as the reagents. The liquid passes through a distributing valve,which opens and closes the different ducts for the liquids anddeterminates the succession of mixing steps. After each measurement thesample-reagent mixture is expelled from the apparatus.

The APP analyzer is based upon drawing a sample into the system butcontains no filtration unit capable of keeping out bacteria; there istherefore a risk of bacterial growth inside the analyzer which again maycause biological activity changing the analyte concentration comparedwith the outside concentration. The sample must be metered preciselywhich appears to be rather difficult with the shown combination of pump,reaction cell and cuvette. A relatively large reagent consumption permeasurement combined with the fastest cycling time (10-30 minutes)results in a time between reagent replacements of about one week. Someof the reagents used may be toxic, and the release of the sample-reagentmixture after each measurement may be a hazard to the environment aswell as to the correctness of future measurements.

The present invention relates to a device of the dialyzer type. Itcomprises a fluid-tight housing having an opening closed by a membranehaving a first and a second major surface and allowing transfer of ionsand molecules between the surfaces, the first major surface in usecontacting the medium to be analyzed, and further includes channeldefining means in the housing fitting with the membrane to define atleast one flow channel delimited by the second major surface of themembrane and by the channel defining means.

Such a device is known from the document AT 355 546. The document showsa sterilizable dialyzer for use in fermentation tanks, chemical reactorsor the like. The dialyzer includes a dialyzer head covered with adialysis membrane. The head is to be fitted in an opening in the wall ofthe tank or reactor. Via a feed line and a drain line in the dialyzer asuitable buffer solution is fed along the back side of the membranewhile the liquid in the tank or reactor contacts the front side of themembrane. Dialysable substances present in the liquid are dialysed intothe buffer solution via the membrane and transported along the drainline to an external analytical instrument or system.

In the invention as specified in claim 1, a fully functional analyzingunit is included within the fluid-tight housing of a dialyzer. Theinvention thus provides a self-contained unit including a carrier fluidreservoir and a carrier pump for generating a flow of carrier fluidthrough the flow channel to allow transfer of ions and molecules betweenthe medium and the carrier fluid across the membrane. As a result, theflow of carrier fluid is transformed into a flow of sample fluid whichis received in a reaction channel. Reagent fluid from at least onereagent reservoir is delivered to the reaction channel by at least onereagent pump, and a detecting device is coupled to the reaction channelfor detecting a reaction product originating from a reaction between thereagent fluid and the sample fluid and for generating a correspondingdetection signal. Effluent from the reaction channel is received in awaste reservoir.

It should be noted that in the description of this invention, the term"sample fluid" indicates a fluid which results from a process ofdialysis. The sample fluid is created by an exchange of ions andmolecules via a membrane; the ions and molecules are exchanged between afluid medium, which is to be analyzed, and a carrier fluid, which istransformed into a sample fluid by the exchange; this is slightlydifferent from common use in the chemical field where "sample" wouldsimply denote a portion of the fluid medium to be analyzed.

This invention avoids or minimizes numerous drawbacks of the prior art.Specifically, the reliance on a process of dialysis minimizes the riskof internal pollution of the analyzing device as well as the risk ofpollution of the environment. All fluids consumed and produced in theanalysis are contained and retained in reservoirs within the housing. Nocontaminant particles or organisms will be aspirated which could disturbthe measurement or cause clogging.

The device according to the invention responds very quickly to changesin the composition of the fluid to be analyzed because the analyzingunit is located within the dialyzer housing, that is, very close to thelocation where the actual sampling by dialysis is performed. The entiredevice may be immersed in the fluid to be analyzed. Reaction anddetection are performed on the spot, and a detection signal indicativeof the detection result is generated. The signal may be recorded withinthe housing for later access, such as in a monitoring application, or itmay be transmitted out of the housing to a remote location for recordingor further processing such as in a process control application.

The embodiment of claim 2 is particularly advantageous in processcontrol applications. The possibility of making a valid detection of thereaction product at any time during an extended period of time allowsvery direct process control. Occasional calibrations and cleaningoperations may be required, but the time interval between calibrations,and the time interval between cleaning operations, may be more than anhour. Deadtime between measurements is minimized, and changes in theconcentration of the analyte monitored are detected with minimal timedelay. Also, the detection, or "sampling" frequency may be adapted tothe rate of change in the analyte concentration.

This is in distinction from batch-oriented methods such as SFA or FIA,in which the detectable reaction product arrives at the detecting devicein batches, which are separated from one another either by air or bysegments of carrier fluid without reaction product. In the known methodsthe output signal, or measurement result, of the detecting device takesthe form of peaks or measurement phases, which occur when a zone ofreaction product passes the detecting device, and which are separated byvalleys or deadtime phases, when a zone of air or unloaded carrier fluidpasses the detecting device. Detection must be synchronized with thepassage of reaction product and timing restrictions are unavoidable. Incontrast, in the method and the device according to claim 2,substantially no peaks or valleys, or measurement phases and deadtimephases, are observed; the flow of reaction product at the detectingdevice is nonsegmented and detection may be made at arbitrary timesduring extended intervals of time. In other words, the repetition rateof the measurement may in principle be increased arbitrarily, the onlyinherent limitation being in the operation of the detecting devicerather than in the flow system performing the handling of the sample.For example, the detecting device may include an analog-to-digitalconverter having a limited repetition rate.

On the other hand, the intervals of time mentioned may be very long andcomparable to or at least in the same order of magnitude as typicalintervals of time over which substantial changes in analyteconcentration occur in large-scale chemical or biological processes,that is, in the order of several minutes to several hours. In otherwords, the intervals of time may be as long as typical time constants ofchange in the concentration of the analyte to be monitored or measured.Thus major changes in the analyte concentration may be monitored ormeasured in an uninterrupted fashion.

The device according to the invention practically eliminates deadtimebetween measurements and minimizes the delay in time between the"sampling" at the membrane and the "measurement" at the detector; theonly delay encountered is the time it takes analyte ions and moleculesto travel through the flow system until they are detected in thereaction product.

It may be advantageous or even required to control the flow of thesample fluid and of one, more or all reagent fluids in the way presentedin claim 3. This depends on the chemical reaction which is to be carriedout in the flow, and on the detection principle to be used. In someinstances it will be sufficient only to make sure that "enough" reagentis added to the flow of sample liquid to ensure complete reaction; thiscan be accomplished by operating with a reagent flow at a safe marginabove a required minimum. In other instances, calibration ofstandardized reactions may require that the volume ratio of sample fluidto any reagent fluid, or between two or more reagent fluids, or both, isprecisely controlled; hence the need to control flow as in claim 3.

It is advantageous to operate with flows having a Reynolds number below5, as specified in claim 4; in such flows axial dispersion is low and ifflow rates are selected to be small on an absolute scale, lowconsumption of reagent fluids is attainable.

Preferably the average volume flow in the reaction channel duringoperation is less than 100 μl/min, as specified in claim 5. This resultsin a low carrier and reagent consumption.

For practical purposes it is advantageous to make the volume capacity ofthe waste reservoir sufficient to allow at least 30 days ofuninterrupted operation; replacement of the exhausted reservoirs will berequired about once a month and may be conveniently planned.

The device according to the invention is especially well-suited foranalyzing contaminated water in waste water purification plants as wellas natural water streams, but it is also suitable for measurement andcontrol of other fluid processes (fermentation, paper manufacturingprocesses, etc.). The invention is, however, in no way limited to theseparticular applications. Any fluid media, gases as well as liquids, maybe analyzed.

It has been found that it is possible with the device according to theinvention to reduce analysis response time to about one minute or less,corresponding to the time it takes analyte ions or molecules to travelfrom the medium to be analyzed, via the membrane, through the flowsystem and to the detector. It is possible to operate the device in situso that the analyte only has to travel an extremely short distance. Forexample, the device may be floated in a partly immersed manner on thesurface of waste water in a treatment basin.

The chemical reactions involved in the detection do not necessarily haveto proceed to termination; if flow is properly controlled, themeasurement can be done at any stage of the reaction since the reagentand sample mixing is reproducible. Other features contributing to thepossibility of very short response times include the ability to attainan increased reaction velocity if the reaction channel is held atelevated temperature, and effective mixing if very small channel crosssections are selected.

As mentioned before it has been found that a device according to theinvention may be constructed to operate for as long as an entire monthor even longer in a self-contained fashion and without the necessity ofservice. The containers holding carrier fluid, reagent fluids, and wastefluid, are all of sufficient size to store the amount of fluid consumedor produced, respectively, over the whole period of uninterruptedoperation which may be a month or more. This is possible because inliquid operation, for example, the liquid consumption may be as low as1-10 liters per month including carrier and reagent liquids andauxiliary fluids such as cleaning agent and calibration standards. Themembrane may have a comparable lifetime if properly selected to beresistant against penetration or invasion by contaminant particles andorganisms.

Application of the device according to the invention as in claim 7 makesit possible to reduce the size of future waste water purification plantsconsiderably because of the quick response of the method and device tochanges in the process conditions which govern the biological processesin waste water treatment plants. Corrective action to any changes may betaken early in time, improving the overall efficiency of the biologicalprocesses and thus reducing the size of future plant or, conversely,increasing the treatment capacity of existing plant. At the same time,the amount and cost of chemicals used in water treatment, such as forprecipitation of phosphate, may be reduced.

The principle of the method as well as the device according to theinvention is explained in further detail below. Reference is made to theaccompanying drawings in which:

FIG. 1 is a schematic diagram of a flow system in accordance with theinvention for carrying out an analysis of orthophosphate in sewagewater.

FIG. 2a is a plan view of a part of a sampling cell for use in a flowsystem as shown in FIG. 1;

FIG. 2band 2c are cross-sectional views of the sampling cell includingthe part shown in FIG. 2a;

FIG. 3 is a plan view of a so-called flow chip baseplate serving as apart of the flow system shown in FIG. 1; and

FIG. 4 is an exploded view illustrating the general layout of aself-contained, submersible device according to the invention forperforming in situ analysis of waste water.

FIG. 1 shows the main components of a system according to the inventionwhich is adapted for analyzing orthophosphate in water. The maincomponents are: liquid containers 1, 5, 7, 10, 13 and 16 for variousliquids 61, 65, 67, 60, 63 and 66 to be used or produced when the systemis working; pumps 2, 6, 8, 11 and 14, all controlled by a controllercircuit 70 via a parallel or serial signal bus 71, for pumping theliquids through the analyzing system via channels 52, 56, 58, 51 and 54;a sampling cell 3 with a flow channel 21 and a membrane 20, in usecontacting the medium 28 to be analyzed, for generating a sample liquid;a so-called flow chip 15 wherein liquids may be mixed in a controlledfashion by proper use of channels 52, 56, 58, 51 and 54 as well aschannels 24, 29 and 59, and a detecting device 12 coupled to the flowchip 15 and to the controlling circuit 70 for detecting a reactionproduct to be analyzed. The detection result is signaled to thecontrolling circuit 70 for display or transmittal via remote signal bus72.

In FIG. 1 the container 1 contains laboratory-grade demineralized water61 which is to serve as a carrier liquid. Via channel 52 in flow chip 15the pump 2 pumps the carrier liquid into the sample generating cell 3.In the cell 3 the carrier liquid is guided trough a flow channel 21along the back side of the membrane 20. The flow channel is defined ordelimited by the back side, or second major surface, of the membrane 20,and by a suitable mechanical device (not shown) in contact with themembrane. The front side, or first major surface, of the membrane 20 isshown to be in direct contact with the medium to be analyzed, i.e. wastewater 28.

The membrane 20 is made of a material allowing transfer of ions andmolecules across the membrane. This will allow the migration of ions andmolecules, including orthophosphate ions, from the waste water 28through the membrane 20 and into the flow of carrier liquid 61. As aresult, the carrier liquid becomes loaded with analyte (orthophosphate)and other ions and molecules from the waste water as it flows along theflow channel 21, which transforms the carrier liquid into a sampleliquid leaving the cell 3 and entering the flow chip 15 via channel 24.Of course, the use of the word "sample" in this instance differs fromordinary use in that the sample liquid in the present flow system is nota physical sample of the waste water, but rather an image of theconstitution of the waste water, formed by the specific mechanism oftransfer via the membrane 20 which may be diffusion.

In the flow chip 15 the sample liquid in channel 24 is led to a mergingpoint 4 where it is combined with a flow of reagent liquid 65 from thecontainer 5, pumped to the merging point via channel 56 by means of thepump 6. The reagent liquid 65 is a mixture of ammoniumparamolybdate(chemical constitution ((NH₄)₆ Mo₇ O₂₄.4H₂ O)) andpotassiumantimonidetartrate (chemical constitution KSbOC₄ H₄ O₆. 1/2H₂O) dissolved in water using sulphuric acid (H₂ SO₄) as a dissolving aid,all in standardized concentrations according to Danish standard no. 291.The choice of chemicals is specific for a standardized method oforthophosphate analysis, requiring standardized reagents and mixingproportions.

The merging of the sample flow and the reagent flow creates a firstcombined flow in the reaction channel 29. While flowing along thereaction channel 29, the sample and reagent fluids in the combined floware mixed thoroughly with each other so as to initiate a reactionbetween the analyte (orthophosphate ions) in the sample liquid and thereagent in the reagent liquid. A reaction product results whoseconcentration increases along the channel 29 as the reaction proceedstowards completion. In the present example the reaction product is acomplex known as phospho-molybdic acid.

At a second merging point 9, the first combined flow is combined with aflow of a second reagent liquid 67 delivered by pump 8 via channel 58;in the present example, the second reagent liquid contains a colorreagent, ascorbic acid (C₆ H₈ O₆), in a standardized concentration.

In the resulting combined flow (which may be termed the second combinedflow), the first combined flow is mixed thoroughly with the secondreagent to initiate a second chemical reaction. In the present example,this is a reaction between the phosphomolybdic acid produced in thefirst combined flow, and the ascorbic acid from the second reagent,which results in the production of a colored species, molybdenum blue,in the second combined flow while it travels along the reaction channel29.

As indicated schematically, the reaction channel 29 runs through adetection device 12. In the present example this is a spectrophotometerfor measuring the absorbance of the liquid which passes through it. Theabsorbance is related to the concentration of colored species in theliquid which is again related to the concentration of orthophosphate inthe sample liquid produced by the sampling cell 3. That concentration isan image of the concentration of orthophosphate in the waste water 28.The whole system may then be calibrated so that the measured absorbanceindicates the concentration of orthophosphate in the waste water 28.

The effluent 66 from the detecting device 12 is collected in thecontainer 16 from which it may be discharged when necessary.

The flow system upstream of the sampling cell 3 may be calibrated at anytime by using specific reference liquids 63 (only one of which is shown)of known orthophosphate concentrations fed to the merging point 4 fromcontainer 13 by means of pump 14 operating into channel 54. Pump 2 isstopped while pump 14 is operated, so as to substitute the flow ofreference liquid in channel 54 for the flow of sample liquid fromsampling cell 3 in channel 24. Otherwise, the device operates in thesame way during calibration as explained previously for sample flow.Calibration for the whole of the detecting device, the reaction channeland the pumps is thus achieved by relating the absorbance measuredduring calibration to the known concentration of orthophosphate in thereference liquid 63.

As an example, if the flow resistance of the reaction channel were tochange, for example because of deposits from the reagents, any resultingchange in the characteristics of the system may be eliminated by such acalibration. In the same way calibration may compensate for changes inpump characteristics from wear.

In a similar fashion, the transmission characteristics of the membrane20 may be accounted for in calibration prior to operation of the deviceby contacting the membrane 20 with a standard orthophosphate solution ofknown concentration instead of the waste water 28, operating the systemas when measuring waste water, and relating the measured absorbance tothe known concentration of orthophosphate in the standard solution.

The pumps 2, 6, 8, 11 and 14 are positive displacement pumps, and thecontrolling circuit 70 is operative to control the feed rates of thepumps so as to maintain a substantially constant ratio between the feedrates of the sample and reagent liquids. Thus a substantially constantvolumetric proportion between the sample fluid and any reagent fluid isachieved at the detecting device. This will ensure that calibration ismaintained.

Also, because the flow rate of each individual pump in the system can beexactly controlled, the time elapsing between the mixing of any volumeelement of sample liquid with a corresponding volume of reagent liquid,and the passage of the resulting mixed volume element through thedetector, can be kept substantially constant. Thus it is not necessarilyrequired that chemical reactions within the system run to completion.Calibration with known standards will ensure a valid continuousproduction of analysis data even on incomplete reactions. This allowsfor very short response times of the system.

A suitable type of pump is described in U.S. Pat. No. 2,896,459; propercontrol of pump operation may be achieved by driving it via an electricstepping motor controlled by a suitable control circuit. Other pumptypes may be used instead, and it may even be possible to usepressurized reservoirs and controlling or metering valves regulating theflows generated.

The flow system may be cleaned, if necessary, by flushing it with acleaning liquid 60 from reservoir 10 which is delivered to merging point4 by the pump 11 via conduit 51. All other pumps are stopped during thisoperation. Both calibration and cleaning of the flow system may beperformed without removing the device from the analysis site.

FIG. 2b is a cross-sectional view of the sample generating cell 3. Thecell includes a channel defining means or support 22 fitted with themembrane 20. The support 22 is shaped generally as a disc which isformed with a meandering groove 25 (see FIG. 2a) on a surface 26adjacent to the membrane 20. Fitted snugly to the membrane 20 as it isin use, the support 22 with groove 25 cooperates with the membrane todefine a flow channel 21 of fixed shape and dimensions which isdelimited by the back side of the membrane.

The surface 26 of the support 22 on which the recess is formed ishemispherical in shape except for the presence of the groove 25. Themembrane 20, on the other hand, is made from plane sheet material andwill become tensioned against the hemispherical surface of the support22 when it is mounted thereon. The tensioning ensures that the membranewill not be lifted off the support 22 by the pressure which prevails inthe flow channel 21 when carrier liquid is pumped through it.

If such lift-off were to occur, the various arms of the meandering flowchannel 21 might short-circuit through the formation of "wild" flowpaths between the membrane and the support. This would entailcalibration difficulties because portions of liquid running through thewild flow paths would be in contact with the membrane 20 during adwelling time which would be different from the dwelling timeexperienced by liquid portions which travel all along the flow path 21.The effect would be that the "wild" flow has generally less time to beloaded with analyte than the "ordinary" flow, causing an apparent changein the calibration of the cell 3. The convex form of the support 22 andthe tensioned membrane 20 prevents this.

The flow channel 21 is shaped to have a rather large surface areacovered by the membrane 20, if compared to the volume of the channel. Byway of example, the groove may have a semicircular form with a width ofabout 1 mm and a maximum depth of about 0.13 mm, resulting in a membranesurface area to channel volume ratio of about 11/mm. Even shallowergrooves may be attainable depending on the elasticity of the membraneand geometry considerations.

The membrane material is selected among materials which in allessentials only allow transfer of ions and molecules across themembrane. This may be achieved by using a membrane made from animpermeable material and subjecting it to perforation by irradiation(such membranes are commercially available under the trademarkNuclepore, amongst others) which will form very narrow channels in themembrane. Other suitable semipermeable membranes are known to workers inthe fields of dialysis and osmosis.

Suitable membrane materials include cellulose acetate, teflon,regenerated cellulose acetate, polycarbonate and polyester. Materialslike ceramics, for example Al₂ O₃, may also be suitable as membranematerials.

Optionally the membrane can be covered with a permeable protectivematrix placed in such a way that the protective matrix is contacting themedium to be analyzed, that is, on the front side or first major surface27 of the membrane. An example of a suitable protective layer is fiberlayer such as filtration paper. Such a coverage may prevent abrasion orother detrimental effects brought about by swelling of the membrane inwater.

The overall thickness of the membrane is preferably about 5-250 μm,especially about 25 μm. Pores in the membrane are preferably about0.01-0.45 μm in size, especially about 0.025 μm. This small pore sizeprevents dirt particles, bacteria, spores of funghi and possibly evenlarge organic molecules from entering the flow system, thus preventingcontinuing biological activity in the analysis system. It is preferredto select the material of the membrane so as to prevent transfer ofparticles from the medium to be analyzed which exceed the size ofanalyte ions or molecules by a factor ten or more.

The support 22 is provided with through-going bores 52 and 24 whichconnect the flow channel 21 to other parts of the flow system. Bore 52leads to the pump 2 for delivering carrier liquid 61, and bore 24 leadsto the merging point 4 in flow chip 15.

FIG. 3 shows a plan view of a baseplate 17 defining part of the flowchip 15.

A major portion of the flow system is formed as grooves in the baseplate17. The baseplate is generally disc-shaped with a central elevatedportion 18 on its front face which is shown in the drawing. A system 50of grooves is engraved in the central portion 18 of the baseplate. Inuse the baseplate is covered with an elastomeric sheet (not shown) whichcovers all of the central portion 18, and the baseplate is fitted intothe recess 30 (see FIG. 2B) in the sampling unit 3 in such a way thatthe elastomeric sheet is sandwiched between the baseplate 17 and thesupport 22. The elastomeric sheet thus serves as a lid or seal for thegrooves 50 so as to transform the system of grooves 50 into a system ofducts or channels. Accordingly, the terms "groove" and "channel" will beused interchangeably in the following description. Connections to thegrooves from other parts of the flow system are generally made via boresextending through the baseplate from front to back.

Sample liquid from the sampling cell 3 is led to the back side of thebaseplate 17 through bore 24 and flows along a channel on the back (notshown) to bore 25. Via bore 25 the liquid returns to the front side ofthe baseplate, where it enters reaction channel or groove 29 at mergingpoint 4.

Part of the groove system 50 are three grooves 51, 54 and 56 extendingbetween the merging point 4 and bores 11a, 14a and 6a, respectively.Groove 51 is connected to pump 11 via bore 11a, groove 54 is connectedto pump 14 via bore 14a and groove 56 is connected to pump 6 via bore6a. Accordingly, those grooves feed first reagent liquid (groove 56),reference liquid (groove 54) and cleaning liquid (groove 51) to themerging point if their corresponding pumps are operated.

The reaction groove 29 extends between the merging point 4 and an exitbore 59 leading to the back side of the baseplate. On its way it meets agroove 58 at the second merging point 9 from which the groove 58 extendsto a bore 8a connecting it with the pump 8. The second merging point 9,being the point where the groove 58 opens into the reaction channel 29,is spaced apart from the first merging point 4.

As will be apparent from the drawing, the groove layout corresponds tothe schematic diagram of FIG. 1. At the merging point 4 the sample flowis combined with the first reagent, and at merging point 9 it iscombined with the second reagent. The reaction products described inconnection with FIG. 1 are developed while the fluids travel along thetwo segments of the reaction channel 29, between the first and secondmerging points and after the second merging point.

The detecting device, which is not shown in FIG. 3, is mounted in theimmediate vicinity of the back side of the flow chip baseplate 17, nearthe bore 59. In proper terms the bore 59 and any additional ductingleading up to the detecting device are to be considered part of thereaction channel 29, as any chemical reactions proceeding while theliquid travels along those ducts may still influence the reading takenby the detecting device.

The grooves 51, 54, 56 and 58 as shown in FIG. 3 each have a narrowportion near the respective merging points and a wide portion near therespective bores connecting them to the pumps; the wide portions aremade to reduce pressure drops. Other details shown in FIG. 3 are formounting and auxiliary purposes which are no part of the invention.

The dimensions of the reaction channel 29 are chosen to ensure that thecombined flow is laminar and has a low Reynolds number Re, preferablybelow 5 in average. This is done by selecting the transverse dimensionsof the reaction channel relative to the viscosity ranges of the liquidsinvolved and to the range of flow rates used in the system.

The Reynolds number Re is defined as:

    Re=(V*D.sub.h)/ν,

wherein V is the average flow velocity, D_(h) is the hydraulic diameterof the channel carrying the flow, defined as D_(h) =4*(A/P) with Adenoting the cross-sectional area of the channel and P denoting thelength of its perimeter, and ν is the kinematic viscosity of the fluid.

As an example, a practical system was realized wherein the reactionchannel was formed as a rectangular groove in the baseplate 17, having adepth of about 0.4 mm and a width of about 0.5 mm which was closed by anessentially flat lid mounted on the baseplate, resulting in a hydraulicdiameter of D_(h) =0,44 mm. The sample and reagent liquids wereessentially water, having a kinematic viscosity ν=1,004 mm² /sec at atemperature of 20 degrees Centigrade. Flow rates in the reaction channelwere selected to be between 0 and about 45 μl/min, resulting in flowvelocities V between 0 and 3.75 mm/sec. Accordingly, the Reynolds numberof the flow varied between 0 and 1.64.

For the detection of other analytes the overall design of the flowsystem as well as the mechanical layout of the channel system in theflow chip 15 will normally have to be specifically adapted to thechemistry involved in the detection. Such adaptations include theprovision of additional containers, pumps and channels for additionalreagents, adjustments of the length of parts or all of the reactiongroove 29 and the spatial relation of merging points so as to allowsufficient flow time for reactions, and other modifications.

FIG. 4 illustrates one of many possible ways of housing and partitioninga device according to the invention. The upper part of FIG. 4,immediately just below a lid 42, by way of example shows containers 16,1 and 10 placed in a reagent compartment 43 to ensure that any leak fromthe containers does not disturb operation or even damage the rest of thesystem. The control circuit 70 for controlling the system and forreceiving/transmitting input and output signals via remote signal bus 72is placed below the reagent compartment. Pumps 2,6,8,11,14 and detector12 are located below the control circuit 70; the sampling or dialysiscell 3 is fitted in the bottom of a common housing 45 which holds allother parts and may be sealed tightly by the lid 42.

Power supply and all communication (input/output) to the system is viathe remote bus 72. An output signal from the system, for examplerepresenting the amount of phosphate in waste water read by thedetector, could be evaluated in a remote control unit (not shown)coupled to the remote bus 72 for controlling a waste water plant inresponse to signals from the analysis system. If the amount were toohigh, the necessary steps to reduce the amount measured could beinitiated at once. In the same way it is possible via remote bus 72 todeliver input signals to the system for example to start a calibrationprocedure.

We claim:
 1. A self-contained, immersible device for analyzing a fluidmedium, comprising:a fluid-tight housing; an opening in the housing, theopening being closed by a membrane, the membrane having a first outersurface and a second inner surface and being formed to allow transfer ofions and molecules between the surfaces, the first outer surface beingfor contacting the medium; channel defining means in the housing, thechannel defining means being joined to the membrane to define at leastone flow channel delimited by the second inner surface of the membraneand by the channel defining means; a carrier fluid reservoir within thehousing, the carrier fluid reservoir holding a carrier fluid; carrierpump means within the housing, the carrier pump means being coupled tosaid flow channel to generate a flow of carrier fluid through the flowchannel to allow transfer of ions and molecules between the medium andthe carrier fluid across the membrane, whereby the flow of carrier fluidis transformed into a flow of sample fluid; a reaction channel withinthe housing, the reaction channel being formed to receive the flow ofsample fluid; at least one reagent reservoir within the housing, thereagent reservoir holding a reagent fluid; at least one reagent pumpmeans within the housing, the reagent pump means being coupled to thereaction channel to deliver a flow of reagent fluid to the reactionchannel; a detecting device within the housing, the detecting devicebeing coupled to the reaction channel for detecting a reaction productand for generating a corresponding detection signal; and at least onewaste reservoir within the housing, the waste reservoir being coupled toreceive fluid from the reaction channel.
 2. A device as in claim 1, inwhich the pump means are operative to generate substantiallynonsegmented flows such that a valid detection of the reaction productis made at any time during an extended period of time.
 3. A device as inclaim 1, including controlling means coupled to the pump means forcontrolling the flow of the sample fluid and the reagent fluid toprovide a substantially constant volumetric proportion between thesample fluid and the reagent fluid at the detecting device.
 4. A deviceas in claim 1 in which the reaction channel has a cross-sectional areaand transverse dimensions, the cross-sectional area and the transversedimensions of the reaction channel being selected relative to kinematicviscosity of the fluid in the reaction channel and to volumetric flowrate in the reaction channel so as to provide a flow in the reactionchannel having an average Reynolds number less than
 5. 5. A device as inclaim 1, in which average volume flow in the reaction channel duringoperation is less than 100 μl/min.
 6. A device as in claim 1, in whichvolume capacity of the waste reservoir is sufficient to allow at least30 days of uninterrupted operation.
 7. The use of a device as claimed inclaim 1 for in situ real-time measurement of plant nutrient salts inprocess waters of waste water treatment plants.